The present invention relates to pixel arrays for MOS cameras. More particularly, the present invention relates to techniques and associated apparatus for compensating for thermal noise variably introduced to individual pixels during resetting of a pixel array.
CMOS image sensors are now becoming competitive with charge coupled device ("CCD") array image sensors. Potential applications include digital cameras, night time driving displays for automobiles, and computer peripherals for document capture and visual communications.
Since the 1970s, CCD arrays have dominated the electronic image sensor market. They have outperformed CMOS array sensors in most important criteria including quantum efficiency, optical fill factor (the fraction of a pixel used for detection), charge transfer efficiency, readout rate, readout noise, and dynamic range. However, the steady improvement in CMOS technology (including increasingly small device size) has moved CMOS image sensors into a competitive posture. Further, in comparison to CCD technology, CMOS technology provides lower power consumption, increased functionality, and potentially lower cost. Researchers now envision single chip CMOS cameras having (a) integrated timing and control electronics, (b) a sensor array, (c) signal processing electronics, (d) an analog to digital converter, and (e) interface electronics. See Fossum, "CMOS Image Sensors: Electronic Camera On A Chip,"1995 IEDM Technical Digest, Wash. DC, Dec. 10-13, 1995, pp. 17-25 which is incorporated herein by reference for all purposes.
CCD arrays are limited in that all image data is read by shifting analog charge packets from the CCD array interior to the periphery in a pixel-by-pixel manner. The pixels of the CCD array are not randomly addressable. In addition, due to voltage, capacitance, and process constraints, CCD arrays are not well suited to integration at the level possible in CMOS integrated circuits. Hence, any supplemental processing circuitry required for CCD sensors (e.g., memory for storing information related to the sensor) must generally be provided on separate chips. This, of course, increases the system's cost.
Despite the advances of CMOS image sensor technology, certain remaining problems prevent their widespread acceptance. One such problem is the noise introduced in CMOS pixels, particularly "passive pixels"; active pixels contain an on-pixel amplifier while passive pixels do not. Read noise on a passive pixel is typically of the order of 250 electrons r.m.s., in comparison to 20 electrons on commercial CCD pixels.
The genesis of noise introduced on passive pixels can be understood as follows. In CMOS arrays, each pixel must be reset after the image is "read" . Generally, this reset step requires that each pixel have an associated transistor switched on to allow a reset voltage to reach the photodiode of the associated pixel. When the associated transistor is turned off, the voltage of the photodiode should be equal to the reset voltage. However, thermal noise in the transistor channel introduce some variability in the quantity of charge injected in each pixel after each reset. Because thermal noise is truly random, voltage variations on individual pixels occurring once will not necessarily occur with the same variation a second time. Therefore, it is impossible to sample the noise at an initial time, store information regarding that noise, and use the stored information for a correction.
Often in the following description, the thermal noise at the root of the above problem will be referred to as "kTC noise" . This is to indicate that the noise magnitude is related to k, the boltzmann constant, T, the temperature in Kelvin, and C, the capacitance in the current path. Technically, the magnitude of the noise is proportional to the square root of the product of the Boltzmann constant, the temperature in Kelvin, and the capacitance in the pixel's current path. The local temperature variations in each pixel give rise to this random noise.
What is needed is an improved MOS image sensor that can compensate for kTC noise.